After the cop pulled my daughter for the brake lites not working, I figured I'd go ahead and refurbish the rear lamp modules on my '86 240. I've always had problems with water accumulating in the lamps...so I drilled small weep holes. I replaced all the bulbs with Sylvannia bulbs from AutoZone. The bulb book referred me to different bulb numbers than what's in the Volvo owner's manual. The Brake/tail light bulb number is 1157 in the Volvo owner's manual; however, the Sylvania bulb number is 7528. BOTH are dual contact bulbs and have 21/5 watts. (specs match up nicely). I cleaned up the units..made sure all bulbs were matched up according to owners manual...the circuit boards were replaced once since 1988...still look good. Now...here's the problem:

The brake lights work good but when I turn on the headlights, the brake lights stay on, i.e., depressing the brake pedal doesn't cause the light to get any brighter....simply a momentary blink. The brake light in the rear windshield works fine with the head lights on. (I know you're already thinkin the wiring harness..but maybe there's something simple I'm missing).

Any suggestions?