If you are uncertain about the wiring on your car, I have a suggestion. Rather than hook up the vehicle battery, purchase one of those small 12 volt lantern batteries and use a couple of alligator clips to connect it to the vehicle battery cables. The lantern battery is capable of generating enough current to allow you to functionally test the operation of all the electrical devices in the car (except the starter); but, it won't generate enough current to create serious damage if there is a short circuit or incorrect wiring some place.

I did a bare shell restoration on my 142 and after I put the wiring back in, I used the 14.4 v battery from my cordless drill to functionally test the electrical system on the car. In my case most of the electrical problems were created by bad chassis grounds caused by a fresh coat of paint.

As to your mystery relay. Do you by any chance have the headlight hi beam flasher function on the turn signal switch? Retrofitting the headlight flasher relay from the 140 series to the Amazon is a not uncommon modification. The headlight flasher relay is rectangular (as opposed to square); but, has more than three terminals, 6 as I recall. If your relay only has 3 wires / terminals then it is not a flasher relay retrofit. Sometimes owners will retrofit a relay to switch the headlights. Otherwise, perhaps as Planetman suggests.