Fitting OBC to an E36

[sniff petrol]

I've got two E36's, one a 1997 325tds daily driver and a 1992 325i track car project that I intend to strip out. The petrol car has a OBC that the diesel doesn't, being as I have no use for it on the petrol how hard would it be to fit in the diesel? Is the wiring already there behind the dash or would I need to wire it in from scratch? If it's too big a job I may not bother.