wiring up rev gauge questions...

[SkunkWeRX]

ive put a gauge cluster from a corolla sedan 88-90?? model into my ae92 s/w all bolted in and clipped in sweet as all works apart from the rev gauge? how do i get it going?
theres a round plug that comes off from round the dissy is this something to do with it???
any help would be great as i put the dash in for the revs basically to keep it looking tidy put its the only part i cant get working.
oh the s/w is a 4a-f carby.
thanks.