So I'm not the only one with an ailing Pershing, eh?
Back to the matter at hand ... Have you thought about putting the speaker inside the tanker? You'd have to hook up some kind of ultra slick contact points for the two wires, maybe in the fifth wheel? Just a thought, but that might make for an interesting project in itself. Or maybe something like this?
https://www.mouser.com/ProductDetail/Ad ... gJG4PD_BwE
At just two bucks it might be worth it just as an experiment. In fact, I might get one myself, just to check them out. At just 1 inch in diameter it should fit right between the frame rails where it will be out of sight but still give good sound if it's pointed at the ground.
But I checked again and they want ten bucks shipping so unless you're buying multiple speakers it's not such a good deal.
I can see if any of the little ones I have here are still working and if so I can send you one, no charge, but even these are just a wee bit too big to fit between the frame rails. On the WC51 I had to put it in on an angle to get it shoehorned in there, so if it fails it's gonna be a PITA to get out of there.
This one is also small enough to fit between the frame rails but is more expensive and only rated for two watts.
https://www.traintekllc.com/tds-high-ba ... gIp5_D_BwE
Trying to get big sound from a small truck is turning out to be harder than it looks. But, there's so much space in the Ural I'm thinking about trying one of the twin speaker jobs. Those speakers are pretty big, at 100x45x20 millimeters, but they do sound pretty good.
When FDR coined the phrase "The Arsenal of Democracy", he was talking about Detroit. Proud to live in the 2nd most violent city in America!!