The W140 like most MB's has a special MB harness that plugs into the back of any MB radio. There are two ways to do this one is to cut the wires and simply run them into the new unit the other is to run new wires to all the speakers amp etc and pretty much build the system from scratch. I think it's better to build a new harness and keep the old one in there that way say if you sell the car and want to keep your custom system you can just plug the old radio back in and reconnect the wires etc.
I think the 94 probably had the two piece radio, with a big din connecting the two, and a fader on the console. The unavailable Becker CD was in line with that trunk mounted tuner.If the car has a bose amp all the speakers are driven off that. Becker has a dash mounted unit with cable adpters, but they dont show it for the 92/94.
I would not trust any of the Best Buy or Circuit City guys , find an experienced audio dealer, it could be a lot of wire cutting
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