So I bought a 325ti auto compact and got a cog light on dash did a scan and come up with shift interlock solinoid so I changed the gear shifter as solinoid attached to it got the cog off and battery went dead charged battery same issue I plug the wires in near the solinoid and shifter won't move take lead off and shifter releases again so garage said to change gearbox filter and fluid as these can cause the problem still have issue someone please help 

Morning James 

I am not aware of a solenoid on the shifter there is an interlock cable but the shift/selector mechanism is divorced from the gearbox. Have a look at www.realoem.com put the last 7 digits of your Vin in the search box it will bring up your model then look at the relevant sections. This is a BMW online parts list so you can make sure you have the correct part for your car. Also look at www.newtis.info this one is a BMW technical information site the same information as BMW Dealers use how too's repair information etc.

The solenoids are are normally in the mechatronic control unit inside the gearbox. ZF advise service intervals of 50 to 80k Km. BMW in their wisdom say "sealed for life" (for life read end of BMW warranty) so depending on how many miles your car has done it really does need a change. if not on miles on age. I assume there were no bad changes or slipping in gears if not then a fluid and filter change may be all you need, however a fluid change cant fix damaged units. A ZF agent or specialist will be able to test the operation of all solenoids and give better advise I would look for a good autobox specialist preferably ZF experienced or agent.

Dave