Oil Pressure gauge wiring on sport dash cluster

I recently picked up a 1989 mighty max with the g63b, manual trans. Dash says 186xxx. I didn't know much about these trucks when I bought it. I was looking for a small pickup to run around town in and found this little gem on craigslist near me in SoCal. When I went to look at it for the first time, I was pleasantly surprised. It had a FRESH paint job (a good one too), after market radio, A/C and heat worked great, no body damage, alarm system....it was sweet. Once I bought it, I did a few things like replaced the water pump, new plugs/wires/coil/distributor cap, the oil pan was leaking so I replaced the gasket on it and got her all sealed up. From what I know now, it has a Sport gauge cluster installed and everything worked great except for the tach and the oil pressure gauge. I just got the tach working the other night by connecting a wire from the negative terminal on the coil to the screw on the back of the cluster labeled IGN -. So this got me thinking "Why wasn't the tach hooked up already? this gauge cluster must be an add on". Now my question is - If anyone has swapped out the stock cluster for a sport cluster, is there a way to get the oil pressure gauge to read on the cluster and how to do it. The truck has an oil pressure sending unit coming off of the filter housing. When I got the truck it had a small sender on it, and I've replaced it with a new large sender, but still no reading. Thanks for the help guys.

P.S. - I posted this in another section that dealt with interior stuff, but got no hits, so I figured it fit in this section and should have probably been here in the first place. sorry, I'm a newb to the forum world.