Seat belt interlock systems have this bypass button mounted on the firewall near the ECU.



Speaking of the ECU, does it have a GOOD ground? I've had issues with the self tapping bolts coming loose. On my wagon, I ran one of those narrow braided ground straps from the ECU to the block itself. Where the strap attached to the ECU, I used a nut and bolt rather than the self tapping bolt.

(not this exact one, just shown as an example)