This was my sole form of transport in all weathers for about 4 years, note the jaunty angle of the rear shocks where I had to move the top mounting to avoid the rot in the frame.
If its this one it checks the continuity of the trailer lights circuit via the testers battery when you press the yellow button, this dosent work on the vehicle side, you just plug it in to the socket and switch the lights on and it checks for a live at each pin.
Here's mine, main part is made from an old commode chair (minus the commode thankfully) I found in the skip at work. Some people will throw anything away :laughing:
I still have, and still use occasionally the SIP Merlin 180 I bought from kays catalogue in the early eighties, I bought it to weld up the rather large holes in my Renault 5 for mot, one old fridge an a tin of underseal did wonders.