You won't find a 20amp open circuit in a car or truck, passenger compartment, it will need to come from the battery or in an '89 from the starter relay post(where battery is connected)
And you need to use a Relay, they are inexpensive and very simple to hook up.
Socket is about $4, 12volt 40amp relay is about $6.
Relay's draw about .5amp, half an amp, so you can tie it into any Key On circuit without "blowing" anything.
Then run a 16 gauge wire with 20amp fuse, from battery to Relay, then wire from relay to Device to be powered.
All automotive relays use the same "numbering" for the connections
Most automotive relays have 5 "posts" you only need 4
30 and 87 are the Load
85 and 86 are the activation coil
87a is unneeded, unused if it has that one
battery--------20amp fuse--------(30)relay(87)-----------------Device
Key on wire------------------------(85)relay(86)-----------------ground