1995 Other Mercedes Benz Models Battery or connection proble

I have a 95 Mercedes C36 which has had a battery drain. The car has not been run recently (replacement of MAF sensor and fuel filter) so when the new parts were installed I tried to start it. Battery was drained - tried charging, but it didn't hold. Bought a new battery and hooked that up. Car turned over and tried to start - no fuel in the new fuel filter, so that took a minute. Then there was a click and a "buzz", and the key alarm sound took on a strange buzzing sound. Turned the key over again, and click, click, click! Thought possibly the new battery needed a charge, so hooked up the charger. Waited a while and tried again - same strange buzz and click, click, click. Unplugged the battery charger and the interior lights immediately went off! Do I have a bad connection somewhere? Have had the battery tested, and it is sending 12.5 volts, with 947 cca (rated at 12 volts and 850 cca)