I have 88T2 and need a speedometer cable. But wondering if I can use an NA there a few @ the local junkyard.

 

Yes.
All second gen MT speedo cables are the same.

 

The S4 MT convertible speedo cable actually has a different part number, which may mean it's a different length. But I haven't seen any actual reason for this under my car, so it's probably still interchangeable w/ an MT coupe's cable

 

do s5's have a cable as well or is it electric?

 

S5 is cable also.