Is this a stock jaguar radio or an aftermarket unit?
Intermittent crackling noises can be a couple all sorts of things, internal and external. Common causes would be earthing somewhere in the circuit or an intermittent short some where. It was a short in the amp part of the output then it would likely blow the amp and you get nothing so you can probably rule that out. Unless you can find a bad contact in the wiring at the back of the radio going to the speakers it is most likely a bad/aged solder joint on the board.
Is there any pattern to when this happens? Going over bumps? Does it do it just as often when the stereo is just turned on or also when warmed up? Are you sure it never happens on the radio or have you jut got lucky and not had that happen yet?
(And yes, I know transistor based stereos don’t “warm up” like older valve stuff but it is in a position behind the engine that can get quite warm!!)