Hardwiring 101 Advanced Level
Every vehicle is different, but I've always been able to find a grommet somewhere to pass a wire through although on some cars the grommets are tough and often nearly chock-full of wires already. But they are the first thing I look for as they are where the designer intended to pass wires through so you can presume them safe for that so long as you pay heed to where your wire runs otherwise. Check both sides before poking a wire through; your chosen grommet may feed into an inaccessible area on the other side! Or you can find a location, bore a hole, then pop in a "body plug" like you see closing holes under doors and uni-body frame areas, running your wire through that. Not too close to heat with these, they will melt is they get too hot but are otherwise perfect for the job. Do NOT run a wire through the common rubber grommets you find on old radios and appliances ("H" shaped when viewed sideways) because they are neither heat-resistant enough to engine bay temps nor are they sturdy enough. And always think of the other side, moving parts, heat, and crash-worthiness.
Another option for low-current items like this is to tap into the power lead going to your radio, cigarette lighter, or another non-critical element of your car wiring from inside the cabin. Either trace and test, or buy a "Haynes" manual for your vehicle model as these have a wiring diagram so you can figure out a better place to tap in at. Most folks use a "Scotch-Lock" wire tap device- those blue things you never knew the name of. You have to match the tap to the wire size- too big and no contact; too small and you damage the car wiring, possibly cutting it. I prefer to solder my taps on; you will know those details already if you have that equipment and expertise. You also need a ground which with all the plastic may not be easy to get on newer cars. Best to seek a bolt or screw where the factory attached a ground. It you're lucky you'll find a "ground bus" plate with an unused spade connection available so you can use a push-on connector.
At this point you may want to consider using this circuit to feed an accessory cigarette lighter socket too, or to use that as the method of powering your dashcam. Just remember to size the wiring and fuse to the planned load and to not overload 'tapped' circuits. Connect the power adapter per it's manufacturer instructions. You're probably wanting to route the wire to your cam in a concealed manner both for appearances and safety's sake. Before you go prying on panels and covers, be sure you're not interfering with anything else as some of today's cars have air-bags in the windshield pillars and you don't want to make alterations which will affect their operation in any way. There may be special tools needed to remove your snap-on covers without damaging them. Here again the "Haynes" manual can help with methods and tools needed. If you can access it on both ends , you might be able to run your wire through the pillar itself, but unless there are wires there already consider that there may be sharp metal edges where you can't see them so use a wire with sturdy insulation for at least that section. The usual thin stuff found on hardwiring kits is not good enough for in-pillar runs but will be OK when used under pillar covers.
If you can't go through the pillar and you can't go under it's cover because of an airbag, don't despair. Get some blue painter's masking tape and lay it 2mm away from the windshield along the pillar. Now do the same on the windshield, but closer. Run a thinnish bead of silicone sealer or even plain latex caulking along this route, then press the wire into it covering with more tape as you go. Choose a caulk color that will blend in with the surroundings, most windshields are 'blacked out' at the edges these days so black works well. If you applied too much sealer it will squish out onto your holding tape causing it to not adhere and making a mess so go light rather than heavy with the sealer. After the caulk or silicone dries peel away the top tape. Now take a very sharp knife and score carefully where the edge of the tape is on both sides, and peel the tape away slowly cutting the sealer again if it seems to be pulling loose. If the finished appearance doesn't look good enough, reapply new edge tape, run more sealer over the wire smoothing as you go with a wet fingertip, let it dry, then cut and un-tape as before. Now your wire is nearly invisible and isn't attached so firmly as to affect the panel's release in an airbag deployment but isn't going anywhere otherwise.
However you do a hard wiring install it is crucial that you fuse your new wire as close to the power source as is practical, then use an appropriate fuse and holder for the environment (waterproof for exposed under-hood). The fuse must never be rated higher than the wire you install, and is best when matched to the load. For most dashcams a 2A or 2 1/2A fuse is plenty. Also crucial is safety of the wire routing so where possible follow the factory wires and secure your wire well, always looking for potential problems as you do that. And you never ever blindly look for a 'hot wire' to tap into randomly because you might be causing problems for a safety-critical system in your vehicle or overloading that circuit. And you never ever alter a car's safety components in a way that will affect their operation.
As one gas station chain used to advertise "Happy Motoring!"
Phil
