These were very simple and sturdy soldering guns [I still have 5 or 6 of them in various sizes] They basically have a transformer wired so that the heating part is basically a short circuit of the large low voltage secondary winding [very low voltage] and because the tip is higher resistance than the rest of the heat producing end, it gets hotter sooner. The secondary is just a single loop of the tube that holds the tip. Usually the only maintenance is to loosen and re-tighten the tip mounting nuts so that there is a good electrical connection [no connection = no heat; poor connection = poor heat]
There may be another small secondary winding that provides power to the light but I think the older ones just attached the lighting wires to the large secondary.
The primary [line voltage] winding of the transformer is switched such that the higher wattage powers the lesser resistance part of the winding and the lower wattage powers the higher resistance part of the same winding. In other words there is a single wire for the primary that has a tap near one end of the winding for dual heat.
If the two windings to the switch are mixed up the higher heat will just be at a different trigger position; if you don't like it, switch them around.

Also check the connection to the non-switched wire from the cord for a good connection. The switches are still available, at least for the newer models; do a google search for
Weller soldering gun switch
or
Weller 7324

Untitled Document [for a switch supplier]

Weller Switch Dual Heat Gun 1 Each 7324