If its a split frame then here is a good link. Not the same loco but similar chassis.
http://www.tcsdcc.com/Customer_Content/Installation_Pictures/HO_Scale/Bachmann/GP35/GP35.html
If it has a plug then any 8 pin decoder with a short harness will work. Bachmann usually puts capacitors in the motor circuit that need to be removed for good DCC control even with a plug and pray.
Pete
I pray every day I break even, Cause I can really use the money!
I started with nothing and still have most of it left!
has anyone put a decoder in this loco, HO scale., how hard is it.
thanks,