The original radio in an '86 does not have a code, it is plugged into a ground wire in the back and will go into lock mode if the power is cut to the radio while the alarm is on. Losing power should not cause this problem, but a bad ground might.

It sounds like your regulator & alternator are fine if it's showing 14vdc at the battery while running, turn on the lights, rear defog, and defrost blower to high and see if it still maintains 13.5vdc or better at ~1200rpm. If lower, the alternator is still suspect, if it maintains this then you have three possibilities:

1 - The brushes are worn (alternator/regulator), and making contact some of the time. I had this failure on my '91 and it was an easy change in a Fire Department garage along the freeway, on Christmas Eve, in a blizzard, with the family in the car, ... Michigan State Police car pushing my 2-year-old car into the bay, ... I pulled the regulator with an offset phillips screwdriver from above, stretched the wires a little to allow the brushes to extend further, put it back in and drove it until the next business day. Be sure you put the hood in the straight-up position!

2 - Bad battery / shorted cell. Batteries can self-discharge or might not even charge properly, it can be tested after a full charge ("load tested"). An Autozone or equivalent will probably do it for free hoping they'll sell you a new battery.

3 - Battery drain. If all else checks out, the light left on or equivalent can drain the battery overnight. A good battery will run the dome light overnight and still start the car, but not a weak one, ... and a proper system will still give a small spark when attaching the battery lead (charging capacitors). If you have access to an ammeter, connect it between the battery lead and post, if there is a constant current greater than ~100mA then start removing the fuses one-by-one to find which circuit is drawing the current.

__________________

Gone to the dark side

- Jeff