One other thing you could do. Run the bypass hose with a selector switch in it that can select the heater core OR the bypass. So one selector valve and a T. Then you can have the heater core for the winter, and bypass it in the summer. The one thing you can't do is just shut it off.
What you need is a 1/2" NPT 3-way valve. Search that on Amazon or google. Looks like 30 bucks for a competent one. Here's an example:
Industrial Grade 1PZA1 Three Way Ball Valve, 1/2...
Basically it hooks up between the heater hoses like an H-pipe in the exhaust, with the valve in-line on the smaller hose, and a hose across to a T in the other hose.