rated current
There is no way that the wiper motor is going to draw 30 amps, ever. That would represent about 360 watts- a huge amount of power, given that we are talking about a simple wiper motor. I am guessing that the amperage is actually '3.0' amps, not '30' amps.
Any 12 to 14 volt DC power supply that can put out 2 or 3 amps should do the job nicely. Probably best to err on the side of caution, and go with a three amp supply.
A simple power supply for this can be built from:
- 110 to 12VAC transformer, rated for 3 or 4 A;
- single diode to half-wave recify the transformer output, rated 50V and 10A. the higer current rating is for momentary surge current.
Connect the primary side of the transformer to 110AC ( a fuse is a good isea in line with one of the wires).
Connect one wire of the motor to one wire of the secondary winding. Connnect the other wire of the secondary to the remaining motor wire, with the diode placed inline.
You don't need a capacitor if you are just ruinning the motor- we're not talking Mars Rover here-, as the motor can handle the fluctuating DC.
Your local library will have acopy of 'The ARRL Handbook', which has diagrams and excellent explanations for building all kinds of power supplies.
Hope this helps