The first thing to establish is the nature of the signals of concern. More specifically you need to establish the frequency range that is being picked up. If the pickup is below 10kHz then you would be better using a ferromagnetic shield. If the pickup is above 10MHz you would be better using a highly conductive shielding material such as aluminium. It may be easier to use double screened coaxial cable for the RF.

Air is a very good "shield" for magnetic fields in the sense that a bigger air gap reduces the field very quickly. Move the RF cables as far away as possible.

For high frequency shielding (>100kHz) you could put the RF cables in a seamless copper tube (as used in central heating systems. These make very good shields due to their high conductivity.

You won’t find usable equations to handle this sort of practical problem.

Yes, if the problem is a magnetic field stainless steel shielding will do nothing to solve it.  There is a material called MU-metal that is an effective magnetic shield.

You can try just re-routing the existing cable, and see if a different orientation makes a difference.  Especially try routing it at right angles to how it runs now.

Also, if the pickup is low frequency and the cable carries RF energy, you can simply buy a bandpass or highpass filter to place at the destination.  The filter will let the RF through, but bounce back any low frequency EMI.

http://www.goodfellow.com/csp/active/static/A/NI03.HTML

I would buy some of this thick foil and position it between the cable and motor, partially enclosing the motor (as much as possible without causing the motor to overheat).