Mine did that on the 85 td last summer so I took one out of a newer 420sel and plugged it in...did'nt work and then one day a couple weeks ago I was sitting in a parking lot and whola it started working.
Then last night found the battery dead, so charged it up. This a.m she went out to start for work and battery was dead again.
Figured it out this evening that the radio was stuck power on with the key out draining the battery.

I Pulled the fuse for now until it warms up. -43 celcius last night...I hope the battery is o.k...on the charger and battery blanket right now. Something wrong with the wiring?