oil pressure guage not working

Just got an 89 laredo for free and the guage is not working. Where should I start looking and how to figure out what is wrong?

Unplug the wire to the sender, needle at the guage should go all the way up, short the wire to ground, needle should go all the way down. If this works, it´s probably your sender (by the oil filter). Wire/connector to the sender has been known to get messed up, when changing a stubborn filter.
The connector at the gauge cluster has been know to mess up. There is fuse for most of the guages. Normally wouldn´t cause just one gauge to mess up though.

Agree with 8Mud. Start by checking the wire and gauge as he suggested. Problem is usually either the wire itself or the sender. If yiou buy a new sender, be sure to buy the one for a gauge -- the one for an idiot light is different and won't work with the gauge.

What's wrong with the gauge? Is it maxed out? If so you've got an incomplete circuit; the sender wire could be disconnected/broken or the sender could have been installed with teflon tape in the threads. Both of those will make it read high.