Once the wires are soldered together, pull the heat shrink from the solar panel wires down to cover the soldered part and any exposed copper.

Leave the heat shrink tube from the voltage regulator where it is for now, the heat shrink must be done in two separate steps.

Use the heat gun to shrink the wrap strung on the two solar panel wires that are now covering the soldered connection.

A reflector nozzle was used on the heat gun, as shown.