A couple of ideas gathered from doing this stuff over the Decades...
Nothing succeeds like success, so the experience of benchwarmer4203
sounds good to me. It's what I've done. Automotive Grade Relays can take the proverbial heat. I've used them for Trailer Brake Duty, etc..
1. If you're concerned about Environmental conditions under-Hood, consider putting the Relays in the smallest, sealed 'Tupperware'-type Container you can find. Or, something similar... All you're trying to do here is copy how Ford reliably mounts Relays on top of the Radiator.
2. Run the req'd +12 VDC to the Relay Coil, and use the Dash/Cubby Switch[es] to complete the Ground side of the Relay Coil Circuit to Chassis Ground. This means the Wire run through the Firewall is carrying only Chassis/Circuit Ground; not +12 VDC. This makes possible electrical shorts a non-issue. This is the way Dome Lights were wired through the Door Switch in the Stone Age. All the Switch did was complete the Circuit to adjacent Ground.
3. Trailer Wire comes as 4 Wire, Color-coded 'Ribbon' Cable. It snakes through tight spots pretty well. Using it means you can ID the different Switched Circuits by Wire color, and have a few 'spares' for any future Electrical Control Projects.
4. Of course, always use a Fuse/Circuit Breaker for the Supply +12 VDC as close as possible to your new Relay 'Bank'. This protects against Mister Murphy and his Law.
I hate re-doing Guano, so I don't. 'Overkill' it the first time, and you're done.
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