My Mercedes Benz 1998 E320 AC is cooling on one side only

A/C is cold or hot?

heater control valve is bad and needs to be replaced.

your air conditioning unit has a gas cannister which eventually runs out. You will need to get this re-filled. Check ou yellow pages or on line for air conditioning services. Prices vary.

you have shutter actuators that are mounted externally on you heater core enclosure inbetween your dash and firewall most likely in center. if not in center then behind glove box.

Have a competent shop look at it. Sounds like a freon leak if previous owner just had it done. You might ask him where he had it done and they should cover it under warranty.

Check if condenser fan is working.
When a/c is on , electric fan on the radiator should come on.
If it's not , system may overheat and shut down , when you move there is natural air flow through condenser , when you stop , if fan is not on no air flow (cooling) through condenser , system pressure rising and safety switch shuts off the compressor.

Try closing the windows, the AC on some Mercedes require all windows to be closed for the side vents to work.

You could need a recharge of freon or your A/C pump is bad. You can go to your local garage and have them recharge with an aftermaket freon kit or the dealer and have them put it on an a/c machine and really diagnose the problem

It sounds like your rear temp. blend door is malfunctioning. You might could get a known good controller from (ebay,junkyard,craigslist) then plug it in. If this does not fix, it is probably a blend door actuator issue.

This link will solve the problem for you

http://74.125.47.132/search?q=cache:YT0z9WoXxCMJ:mbworld.org/forums/e-class-w211/250487-diy-installing-new-changeover-heater-control-valve.html+2004+%22E500%22+mercedes+rear+vent+heat+front+cold&cd=8&hl=en&ct=clnk&gl=us