If the '86 cannot be brought back online with a four digit code (entered with
the preset #s on the faceplate of the radio?)

Connect a second battery with jumper cables to the positive and negative
(appropriate in each case) clamps of the VEHICLE'S battery cables.This will
maintain power to the radio set while you replace the defective battery.

This is a kinda tricky operation AND you want to be ABSOLUTELY sure of
correct polarity (s).

ALSO: you might want to take some electrical (or any insulating tape) tape
and secure the jumper cable clamps to the battery cable's clamps so that
there is no possibility of: 1. Grounding the Positive cable to ANYTHING. 2.
touch contact between the Negative cable to Anything.

Wrap each cable clamp and it's jumper cable clamp up like mummies.

Yeah, It'll cost you a roll or two of tape.

Reconnect the appropriate cable clamp to the new battery terminals ...
Remove the jumper cable's clamps from the "Helper" battery (Negative
first)
remove the mummy swaddling and the jumper cable clamps from the
vehicle's cable clamps (one at a time,Negative first)