Simple test is to remove negative battery cable from battery, connect a test light between battery cable and battery post, if power is being pulled from battery test light will glow, you can use a Volt Meter as well, light is just easier to see.
Leave key off and out of ignition switch, and of course doors closed, i.e. no dome light on, and if you have a hood light pull that bulb or it's connector.
1990 should have the Power Box in the engine compartment with fuses inside
Start there by pulling out 1 fuse at a time and watch the test light, light will dim or go off when you find a circuit that is using power.
Pretty much all the power coming from the battery runs thru this power box, so you will find the circuit draining the battery, but it may be on a circuit feed from inside fuse panel so you will need to narrow it down more.
There are 2 or 3 fuses in the Power Box for the inside fuse panel.
Radio/clock often pulls a little power so pull that fuse first if you have found the power draw is on a circuit fed from that inside fuse panel.
Then pull 1 fuse at a time until test light goes off.
An alternator can draw power if its ON/OFF circuit fails, it is just an electric motor used in reverse.
So with test light set up, unplug the 3 or 4 wire connector from the alternator see if light changes, you could also pull off the B+ wire on the back, the large terminal with large cable and nut, be careful with this wire, it is live.
If you go here: http://www.therangerstation.com/tech...rams/index.htm
And then scroll down to the bottom you will find the Power Distribution wiring diagrams for a 1991 Ranger, should be very close to yours, it can help you ID fuses and circuits