Classic/Antique Car Repair: 59 Cadillac, battery cables, copper core

Question
1. starter turns slowly (like a weak battery) when engine is warmed up (not overheated)?

2. wiper motor runs in 2 speeds but will not turn off?

Answer
All cars do this to some extent, because the copper wires in the starter and in the battery cables have increased resistance at higher temperatures.  You hardly notice this effect on most cars, but you must have a further problem.  If your engine has been recently rebuilt, the machinist probably didn't align bore the main bearing saddles - this shortcut will cause the crankshaft to bind up a bit when the engine is warm.  This will get better as the bearings wear a bit.

Your battery cables may be too light for the car - the copper core of the cables should be at least 5/16 inch thick - if someone has put cheap cables on it, that will contribute to the problem.

Your starter may be dragging (the armature rubbing on the field pieces), the only cure for this is to have the starter rebuilt by a competent rebuilder.   If you have a high current ammeter (400 AMP), or can get one, check the current drawn by the starter - if it is over 250 AMPs or so, your starter needs to be rebuilt.

As for your wiper problem, there is a wire which provides 12 volts to the wiper motor when the switch is turned off, that  is needed to park the blades and then turn off the motor.  If the wire falls off, or the switch isn't wired right, you have the symptom you report.

Dick