Here are the lessons I learned during the process of getting my Honda outfitted for flat-towing behind my Interstate:
1 - Even though the Interstate has the turn signals separate from the brake/taillights, the 7-pin connector is wired for combined brake/taillight/turn signals on one dual-filament bulb, so that's the way the car has to be wired, even if the car also has separate turn signals.
2 - The wiring kits don't include wiring for lighting up your car's rear license plate when the taillights come on. You've got to add a separate relay and wiring for that.
3 - Invisibrake has a cutout timer; if you hold the RV's brake for too long, the Invisibrake cuts out until the next time you press the brake. Not a problem as you should already be at a complete stop by the time the cutout cuts out. However, it played merry hell on the technician who was installing the system and then scratching his head as to why the Invisibrake suddenly quit working when he was checking all of the connections.
4 - When installing the Invisibrake under the driver's seat, mount it so that when the seat is slid all the way forward, you can see the dial and reach the adjustment knob, but when the seat is slid back to your usual driving position, the knob and dial are reasonably well protected from a rear-seat passenger's feet. Actually, I figured that one out before the installation, so nothing had to be redone, but it seems worth mentioning.
Engineering: Finding complex solutions to simple problems you didn't even know you had.