I think, although I am not completely sure - that the ground wire of a 1978 Tomos or Tomos-Koper is actually connected to one of the two bolts that keep the headlight itself on and in place.
This sounds like the wire would be loose, or so oxidized that it's not getting connection, and there is also the possibility of the lead wire being loose.
Only two wires go to the older Tomos headlights, and it's usually ground wire to one of the mounting bolts, and lead wire directly to the back of the headlight rear cover itself, usually just a bit lower than dead center.
Check that connection first, just pull it, and if it comes off to easy, you know it was that one - sand the connector or wire brush it a little, and crimp it just a hair more with a set of pliers, then reconnect it - it should be stiff and not easy to remove.
To check the ground wire, the simplest way is to remove the two bolts holding the headlight on from the sides, under one of them will be the ground wire with a connector that the mounting bolt goes through....it's possible, but difficult, to get it all lined up again removing only one bolt, so best remove both.
That connector can be gently sanded or wirebrushed with a little coca cola to remove oxidation (that iccky green stuff) and just bolt it all back together firmly.
Usually it's not a wiring problem as much as it is a connector problem, but if you try alla that and still have a problem, or a DVM read tells you that there is a break in the wiring, let us know, cause there is a simple way to do the re-wiring, but I'll save that fer when it's needed.
FYI - Tomos uses standard 12V Auto Bulbs, for the most part, available at any gas station or even 7-11.