Bill:

I back in the late '80s I a 3rd rear brake lignt to an '85 245 M46 which came as a kit from Volvo. As I recall the power connection was a line that ran from the + feed at the 3rd light to a 2 pole male spade connector at the brake light switch at the brake pedal. The gound went to a screw under the rear hatch cover. By feeding the power from the 2 pole spade brake light switch there is not out of balance draw from left or right to cause the light failure light to come on.

Of course you have to bring the line through the tailgate hinge down the right rear pillar and under the floor up to the brake pedal. NAPA sells the single to dual pole bronze connector adaptor that you need to do this.

Hope this helps

Al