86 and 87 will have different wiring harnesses. The cab itself will be similar, as far as metal goes, but the easiest way if you plan on using a 86 cab (I assume you have an 87 and for some reason need a new cab), is to swap the wiring out from the 87 to the 86 cab you are planning on putting on.
If you do not have the complete 87 cab, then the next easiest would be to get a complete wiring harness for a 87 and put it on the 86.