I remember I have read about technology for soldering wires: Remove
the insulation and twist them together. Wrap the twisted wires with
thin solder (i.e. "wire" containing the alloy and flux). Pack all this
into aluminum foil tightly. Heat with a gas lighter for several
seconds. The aluminum foil helps to distribute heat, prevents the
solder alloy from dripping, the copper from oxidation and the flux and
the wire insulation from burning. It can be removed after the
connection cools or it may be left where it is.

BR from Ivan OK1SIP