It's simply low on charge. It's not empty, though. If it was, the compressor wouldn't cycle at all. You have a minor leak (so slow that it has taken 20 years to get low enough), so you just need a top up. However, your 93 still uses the old R12 (unless it has been converted previously, look on the accumulator for an adapter fitting), so you will need to find a garage that has R12. If it HAS been converted, you can buy a refill can of r134a at any store and fill it up. Do NOT put R134a in it if it hasn't been converted (hard to do since the fitting sizes are different). In my experience, your symptoms (compressor cycling quickly, no cooling) will require a full can. Fill with the AC on MAX, fan on high, RPM at 2500 (again, only if it has been converted).