It may be your Central Locking Vacuum Pump, located under the passenger side rear seat or under side trunk panel (same side).

I'm not sure if it's compatible but I have a good used one from my W202 C280 model. It was an extra from troubleshooting an issue that I found wasn't the pump just disconnected lines.

just an example not actual part or correct model
I can get a photo of the one I have, if interested
email me at euro.sport.fan@gmail.com


For your electrical issue
check your OVP over voltage protection relay
located near the battery (it will have a fuse on top)

but also could be a deteriorating wire harness issue
common with most old Mercs