Melting plastic housings at the headlight are probably due to light
corrosion on the headlight/bulb contacts themselves.  As corrosion increases
at this connector, the heat generated locally because of the increased
resistance caused by the corrosion melts the plastic.  The power dissipation
across the connection is the product of the square of the current times the
resistance.  If your headlight draws 7 amps and the resistance is even as
small as one ohm you will have a 49 Watt plastic-melting heat source.

Periodic maintenance might be as simple as removing the connector, shining
up the bulb contacts (all three) and slipping the connector on and off a few
times to clean up the inside of the spade female.  Punishing, I know, but
accurate.  The wiping motion of sliding the connector on and off several
times helps to make a better contact.  Silicon grease doesn't work very well
here as the normal heat from the bulb/reflector assembly can liquify it.

p