1950s Wiper switch issue

[FleetwoodMark]

Recently installed (with grounding strap secured to firewall) rebuilt wiper motor in my 1959 Cadillac Fleetwood Series 75. Also had to take out wiper/washer switch to inspect the components as it wasn't working.  The wiper/washer switch worked (not perfectly with regard to wiper speed-seemed same speed at any setting. Also the motor rhythmically clicked).  Upon reinstalling the switch into door panel the wiper motor activated when an installation screw contacted the metal car frame. The motor ran much quieter when activated by the chassis ground.  I have very little experience with electrical and have no idea what's going on. Any advice out there?