I have done this before on another kind of car.
Drill a hole in the firewall in a SAFE place. Insert a grommet into the hole.
Push new wire through the grommet and connect like you want.
Test the new arrangement.
If everything works, then cover the grommet with silicone to prevent water/dust from getting through.

If you don't want to drill, you can try what you were thinking about. As long as the space is not too tight.

 

Just another thought... is there a connection in the hood area?
I remember one time the connection was bad. This was one of those 15 wire cable and I simply found the right 2 wires and spliced the wires to get around a bad connection.

 

Just got back in, I got it working. I ended up splicing under the dash and ran the new wire thru the firewall grommet and then spliced at the relay connector. Bad ground in that 5' section.

Thanks for all the advice, really helped me out.

 

Glad you got it fixed. Keep in mind that the plastic wrapping on the wire harness is likely damaged and has one of the red/blk wires shorting to ground. It is possible that you have another wire eventually short to ground. You may want to follow the wire harness to see if a holder is loose or the wire harness is touching something metal in that 5 ft section. It is not urgent, but something to keep in the back of your mind.

 

Thanks, that has crossed my mind. Unfortunately, I didn't find any obvious damage. The small section of wiring it's in is pretty much inaccessible without removing major engine components. Just going to keep my fingers crossed and hope I don't have to deal with this mess again, was not a pleasant experience, lol
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