Replaced battery and radio/tape player won't work, 92 525i ?

[Frankabq]

I changed out a dying and dead battery under my passenger side back seat today. Now the radio/tape player does not come on. Any ideas where the ground is or if I need to have some thing reset in the computer?

There is a small box at the end of a grouping of wires from a cluster on the drivers side under the back seat that led to the other side by the battery; that had a bracket that has a screw hole in it, looks like it might fit under the gold battery bracket and fits the screw that holds the bracket and battery in place secured to the floor board. I am pretty sure the black box 1" x 2" x 1/2" was not secured this way before I removed the battery. Could this be how it grounds the radio/tape player and I was just lucky it was grounded some other way before? OR is there some thing else that I missed?
Any ideas that could help.
Thanks, Frank.

P.S. I don't know if this matters but I have the 2.5 liter M50 engine in my '92 525i 4 dr sedan.

[whiskychaser]

A pic of the little black box would be handy. But if it has got a silver lever on it that operates a switch inside the box, its part of the security system. It should be secured to the top of the battery cover. The switch lever points upwards so it is closed when you put the back seat down. Anything else not working - interior lights maybe? Have you checked fuses, particularly 18 and 41?

[632 Regal]

Any time the power is off you need to reset the radio code, without setting that it will always seem to be dead. Get your code and read the manual how to set it, real easy. Turn key to run position, enter 5 digit code on the radio and good as new. Sounds like the little box deal in back is the remote alarm, has nothing to do with the radio.

[Frankabq]

Thanks guys, been so long since I have replaced the battery I forgot all about the radio code. I even have it written down in my old glove box user manual, LOL. Just finished fighting with the fan and shroud after installing a new thermostat and housing twice and wasn't thinking straight.
Thanks for the memory jog.
Frank.