1992 lx 5.0 park it for the evening and the battery is dead by morning? Interior is removed so no lights can be left on, alternator is charging at 13.7-14volts. Any ideas or possible causes? The battery is less that a year old optima red top.
Obviously there is a draw. Hook up a DVM. start pulling fuses one at a time and see if the voltage changes. This is usually from a hacked up stereo, alarm install, fan. Check for remnants of wiring under the hood and under the dash. Also do a voltage drop test starting from the battery back, and check the ground wires for chafing, or grounding out.
Another useful test is to get a DMM that can measure DC amps up to 10A. Remove one terminal and hook the meter up in series and see how many amps of current is being drawn. Should be 1/2A or so. If it's high, start pulling fuses to identify the circuit that is drawing current
I would add that it might be useful to use a shunt or test-light initially - your draw is large and it might be a bit much for the DMM (one doesn't want to test the circuit protection in the meter itself).
Typically it is something draining the battery. Small things like glove box or courtesy lights are often the culprits. If you have an aftermarket stereo or alarm system, it is also suspect.
The ideal method is to disconnect the negative terminal, and connect a Digital Multimeter (DVM) between the negative terminal on the battery and the negative cable. Set the DVM on a low current scale of 2-5 amps if it doesn't auto range. Watch the current draw, and then start pulling out fuses. When you see a sudden drop in the current, that circuit is the likely culprit. Note that the computer, radio & clock will draw less than 1/10 amp to keep the settings alive.