caravan sound system wiring

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Paul,

The new player is unlikely to draw much more power than the old radio unless you have really screwed up the wiring.
It seems more likely that your battery is very low and with the player and lights on it is draining to a voltage low enough to cause the player to drop out.
A low voltage may also be the reason for the antenna not working.
Do you have a meter to measure the battery voltage?

Some radios used a common negative for the sound speakers, ie, bothspeakers ran to the same neg point in the unit.
Others have two wires dedicated to each speaker and therefore separate neg lines back into the output section of the amplifier. There may be such a conflict with your setup.

I discovered older Mitsubishi vehicle units used three wires for two speakers and Fijitsu Ten used the 4 wires for two speakers, 8 in total for 4 speakers.

Although wired and working assuming the negatives go all to the same point may be a problem. A mate wired a Fujitsu Ten player into a Mitsu and it used the three wire idea, not 4. The heat sink on the new player ran extremely hot and it used to cut out with the heat developed and it was drawing far more power than normal.

The cutting out may be the unit trying to save itself. Does it get hot on the heat sink?

As I assume you keep your van batteries charged when parked up, again assuming your parked, the diming lights doesn't sound good.

I would be checking the wiring carefully, as RMD wrote above above check that a separate speaker wire have been run to each speaker as grounding a speaker wire or connecting to another channel can destroy most modern radios.

Turns out I neglected charge the battery after moving the van with the electric jockey wheel.
Oh well!