18v charging and 12v radio

Customer comes in for a mechless deck and speakers in a 72 jeep. Goes on down the road and comes right back saying he smelled smoke. After replacing the 5a fuse on the deck there is no audio. Casually mentions he has an unregulated alt. After some testing I find that upon increasing rpm the radio harness is peaking around 18v(sustained),idling is around 14v. Any options to keep him from frying another radio? I fear the answer may not be inexpensive. TIA

Yes. Fit a regulator. Better still, upgrade to modern all-in-one alternator regulator if it's an old external type (preferably 2-wire S&L type; see my other alternator posts). An unregulated alternator indeed!! How many batteries has he fried or when will the battery flame or explode? Not to mention wire and other corrosion, lamp failures, and other 12V appliance failures. At least this is solely the customer's fault else responsibility. And though there are other solutions to your question, IMO it would be risky or irresponsible doing such on a system that is known to be hazardous or is likely to be a hazard.    IMO you should merely fulfill your (assumed) responsibility and paraphrase the above to him. PS - feel free to include my professional consulting fee in his charge (please!). Payable to you as per normal of course!

On the Yellow wire of the radio, get (10) 6 amp diodes and wire them in series. Place this array of diodes in series between the vehicle and the Deck's yellow wire. You will see a (.)4 volt drop across each diode.   For the red wire of the deck, build a simple voltage divider circuit. (4) 470 ohm 1/4 watt resistors wired in series. Vehicle's switched wire to one end of the array. Ground the other end. Connect the Radio's red wire to the end of the resistor that is connected to the vehicle's switched wire. Not at the point that the switched wire is connected, but the other end of that resistor.

Wouldn't the diodes and resistors overheat the more the engines rpms go up? I am inclined to tell him that without getting the alternator fixed he will go without tunes or else keep buying decks every other block he drives. A sidenote this is a inexpensive planet audio deck with a 5a fuse. The fuseblock has a 20A for the radio circuit but it does not blow. I have the impression that even if I had a larger fuse in the radio the problem would still persist right?

Never replace a fuse with a higher current rated device. The resistors will not heat up. Neither will the Diodes.

The resistors should only dissipate ~0.2W each, and the diodes ~2W each, maybe 3W. It's not the proper solution as I wrote earlier. His batteries will soon cost more than the proper fix. And never exceed standard fuse ratings. That requires a downstream wiring & connector check (including ground) else new cabling to handle the increased rating. Failure to do so probably means loss of the vehicle thru a cable melt & fire quicker than the 18V will cause a battery or engine-bay fire. I'd assume the over-voltage has fried his audio. It will do the same for other things that are exceeding the normal 16V "12V system" design limit. Fix the alternator! Or IMO suffer the list of further premature electrical failures.