12V 20A: What to do for high heat
You must be sure to select an adequate wire size for the current considering the temperature in the area where the wires are located. Also make sure that the wires are adequately protected from mechanical damage. Make sure that the wire connections are enclosed or otherwise protected from coming in contact with anything that would cause a short circuit. The connections must be tight and connected from corrosion. There must be protection from excess current due to motor overloading or short circuit.
You should probably use 14 AWG or larger depending on the insulation. For a vehicle, you should probably use finely stranded wire. If you use a high temperature insulation, you can use a little smaller wire, but you may need to be concerned about the mechanical strength of the wire and insulation.
I would be inclined to use 16 AWG with 27/30 stranding in silicone insulation covered with fiberglas braid. Or perhaps this: https://www.amazon.com/BNTECHGO-Silicone-Flexible-Strands-Stranded/dp/B00TG1TRL2/ref=sr_1_12?dchild=1&amp;keywords=silicone+insulated+wire&amp;qid=1594775032&amp;sr=8-12
Using high temperature insulation allows a smaller wire gauge to be used. That offers the advantage of leads that are easier to work with, lighter weight and more flexible. The disadvantage is that any material that comes in contact with the wire must be capable of withstanding that same temperature without damage. Silicon insulation is rated 150C to 200C. SEW seems designate 200C silicon with a braided fiberglass outer protecting layer to abrasion resistance. Teflon insulation is rated 200C to 250C. TGGT seems to designate 250C insulation with a fiberglass cover. SRMC seems to designate 150C silicon insulation with no outer cover. Silicone insulation with no fiberglas cover would have better abrasion protection.
The Amazon product linked above has 200C silicon insulation with no outer cover. It has very fine stranding. The combination of very fine stranding and no outer cover makes it very flexible. I mentioned it because it seems to be easily available in sort lengths.
I would think that the motor controllers could not withstand the stall current for more that a minute or so. You might want to use 18 AWG wire for mechanical strength, but you will probably not get it above 150C. You probably don't need to select the wire based on heat. You will need to decide if you need the best abrasion resistance and how difficult it is to get the total length wire that you want to buy.