lights are real simple with the right gimic
a wire rapping clip with a little prog to go thru the existing wire
for brake, running tail, or turn ,
most lite sets allow you to pick two not all three
looks a little like a split ''T''
no need to cut or splice they just snap in place
with a plug to joint to the trailer plug

now anyone got a pic of a proper 126 trailer recever/hitch
as to where the pro's think they should be mounted???
I trash picked a fairly flat bar type recever but will need to extend it a little
to fit my 126 300sd 83

I was spoiled by Volvo's they all have built in mount points
pre threaded even and hitch/recevers are common and cheap
and others cars/trucks units even are not hard to bolt up to the mount points
I had a class 5 unit from a econline on my 740 wagon
Northern [china] tool switched brands and dumped the old brands
recevers for about scrap value