Hey, I had the same problem a few weeks ago. I took it to the mechanic and after 2 weeks the guy found out what was wrong with it, apparently the cheap ass becker radio was not turning off properly or something and was draining the power over night.
Since you just installed a new cd player, I would definitely say it's that if your problem started after installing the CD player.
My car is working fine now after disabling the beker radio. i've got to get rid of this junk.
87' 300E champagne - Euro headlights and corners, 8-hole rims, 140,000miles/220,00km.