There was a unit called a Lucas 2TU there are some I see on ebay which was designed to get around the BMC cars of the time that had duel level brake lights designed not to dazzle at night, it had relays for indicators and brake lights to remove load from car rear lights, and also included a fuse.
Today I would used the Durite 0-727-14 relay or similar, there is a version with built in fuse if required.
The 7 pin socket has changed what pin 2 does, it was interior light in caravan unless using vacuum brakes when it was vac warning, then caravans started to have electric operated reversing hitch so it became reversing light, then the illegal rear guard fog lamps became a legal requirement so then pin 2 became rear guard fog warning.
This pushed the reversing light, brake warning, and power to a second plug, now both combined to a 13 pin plug.
There was also special flasher relays, two types, the two pin type you fitted a second of higher amp rating can't off hand remember how wired, but the new relay would not flash unless correct load, the other type had three pins and was replaced with a 4 pin model so there were two warning lamps one for caravan and one for car.
There were other options, which copied the larger bosch type, at that time having a buzzer to show when caravan indicators were working was not permitted, it had to be visible, I don't know if that is still the case? There was also a warning light relay that measured to current and if enough flashed the warning lamp, as an add on where original flasher worked OK with extra load.
Split charging never really worked very well, however some modern caravans have electric anti snake devices and need power, also in some cases the fridge also works from car. The fridge and split charging are not permitted to work from same supply, but want both to only switch on with ignition, so in some caravans there are relays in the caravan so the non ignition supply is switched on when the ignition supply is made live.
Today it is getting more complex, with the switching off of alternator causing a problem with anti-snake device so the old voltage dependent relay can't be used any longer, we are back to blocking diodes to ensure when car is started current is not drawn from caravan battery. But with your old car that should not be a problem. However it has resulted in a new device, the DC to DC inverter for charging the caravan battery.
I think for 164
Maximum towable weight 1300kg (that would be 2866 lbs).
Max. vertical load on towing hook 65kg (143 lbs).
where the X-trail was 1500 to 2000 depending on model so hope it is man enough?
