Should I d/c battery to pull k40? (ABS/no brake lights) RE: W210 ('96 e320) M104
 

And if so does a 9v battery "settings keeper" work OK with these sensitive cars (assuming I plug it into a lighter plug that stays hot(+)?

YES I have replaced the stoplight switch, tested the new one (old one had dead spots).

Long description of intermittent ABS/brake light problem follows if you're curious

Latest symptoms:
Yesterday, I started car (voltage fell to appx 9v while cranking, (digital meter, no logging feature so hard to tell), all OK
.
Put in gear; A/C, high blower, rear defrost, headlights/hibeams on, 13.5v steady. waited 2 minutes or so, putting brakes on & off. Still OK.

Pulled fwd 20ft, put in park, step on brake still oK. Shut off, restarted, still OK.

Backed out of drive drove 2 mi, including many stops, 50mph, and 90 degree turns, still OK.

parked, restarted, still OK,

2hours later, we need the car. Start up, Step on brakes - dash warning light festival: ASR/ABS/LAMP DEFECTIVE and no brake lights, pumped brakes on&off (to maybe loosen up new brake light switch(?) turned wheel lock-to-lock, (reset steering position sensor), restart car - no change.
Took other car.

Tried it 2 hrs later, at first FUBAR (incl brake lights not working), then I pressed/released brake pedal as before, noticed the brake light working (not intermittent and with AC/headlight load on, 13.5v). Shut off car, restart, all OK.

Is this car just LAZY, or trying to drive me nuts??