Well, interesting to me, frustrating to you. I don't think it is the switch at the brake pedal but rather something in the cruise amp (the board that you resoldered) that is causing this problem. My suggestion is to again remove the board and inspect it carefully for any solder bridges you might have accidentally made while resoldering the board or any points missed. I know from my own experience resoldering this board (twice) that it is easy to miss one and that's all you have to miss is one.

My schematic of the wiring that is common to the brake lights and the cruise control shows pretty much what you have described -- one wire to one pin of the switch on the brake pedal (brings +12 Volts from the fuse panel) and three wires tied together on the other pin. Of these three wires, one goes to the cruise control amplifier board, one to the cruise control actuator (the mechanical thing that operates the throttle), and the third to the brake lights. There's nothing in the actuator that would "latch" off so it has to be in the amplifier.

Jeremy

__________________

"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970