Your 7-way flat connector is unfamiliar to me, but the round type appears to be exactly the same as our "12N." Do you know the pinout arrangement?
We used to have a 5-way round connector at one time for basic trailer lighting (tail, left turn, right turn, brake lights, ground), until it was replaced by the 12N arrangement in -- I think -- the late 1960s/early 1970s.
Here are our connections/color code for 12N:
1. Yellow = Left turn signal
2. Blue = Rear fog light
3. White = Common return
4. Green = Right turn signal
5. Brown = Right-hand tail light
6. Red = Brake lights
7. Black = Left-hand tail light
Pin 2 was originally spare or used as a permanent 12-volt feed for caravan interior lights etc. Then in 1980 the "powers that be" dictated that all new vehicles had to be fitted with a rear fog lamp, so it was reallocated for that.
The split left/right tail light feeds are apparently part of the European standard, as European cars fuse them separately. (Many European cars also have a curious arrangement whereby when the ignition is off the turn signal switch allows you to illuminate the tail/parking lights on just one side
of the vehicle only. Maybe Wolfgang can elaborate on that.)
The 12S connector is similar to the 12N but has the male/female on the center pin reversed (and, as mentioned, the standard 12S cable is heavier gauge). The 12S standard was
1. Yellow = Reversing light/surge-brake solenoid
2. Blue = Battery charge
3. White = Common return
4. Green = Permanent 12-volt feed
5. Brown = Sensing device
6. Red = Refrigerator
7. Black = Spare
Pin 2 is wired through a relay so that it is powered only when the engine is running and the generator producing output (ditto pin 6).
Pin 5 is allocated for a remote sensing device on the caravan/trailer, but in practice is normally unused.
The 12S standard was revised slightly in 1999. First, it was decided to provide a separate return path for the fridge power on pin 7 (black). Second, the newer electronic caravan charge units now don't rely on the switched power on pin 2 for charging, but instead just take power from the permanent feed on pin 4 and connect automatically when the appropriate increase in voltage is detected.
That means that pin 2 is no longer needed for battery charge, which is fine. Unfortunately, some dimwit in the standards committee (presumably) decided that pin 2 should be re-assigned to the function of indicating to modern car electronics when a trailer is connected. And how does it do that? By having the trailer wiring ground pin 2!
So, post-1999 caravan + pre-1999 12S car wiring = Blown fuses!
(Or judging by the number of horrible lash-ups I've seen in cars where the charge relay is connected straight to the battery positive with no fuses.....
[This message has been edited by pauluk (edited 07-31-2006).]