Very simple to do. My '91 had the heater bypassed when I got it due to a leaking core.
If you look at the pass. side valve cover you will see two metal tubes running towards the back of the motor, one of them having a tee with a screw-in cap on the top. They have the heater core hoses connected to them about 2/3 of the way back, simply disconnect bpth hoses and run a new piece of hose in a loop from one metal tube back to the other. After you've tightened the hose clamps you need to remove the screw-in cap from the tee to bleed the air out of the system as you refill it.
And then start saving your pennies for an expensive heater core, and psych yourself up for what is a less than pleasant job to replace it. Took me about 4 hours to do mine.
1991 RRC
1989 RRC "Parts rig"
1961 LR SII 88"
1967 Sunbeam Tiger
1970 Mercedes 250C
1989 Ford Ranger 5.0 H.O.
1968 Ford F250 CS