Searched on this but not really finding the exact same issue.

I have the problem where the trio of BAS, ASR, and ABS static warning lamps are triggered every time I start the car (98 E430), and as soon as I reverse out of the driveway, the warning messages for all 3 are cycled through on the display (with related beeps).
Based on previous advice, I went ahead and replaced the brake lamp switch under the dash myself, but the warnings happily came storming back as soon as I started the car again.

I've seen mention to clean some sensors in the rear, though I dont know how to locate these. What else should I try?