Tacoma Bed with cap climate control for dog concerns...

Leer cap with a front slider, get the window boot they make for it. Run you AC back to the cap by opening both sliders. They also make screens for pets. Get the carpet liner which also helps insulate the back. When you get a new truck tell the dealer you want a cap installed, then it will be in with your payments or if you have the money buy it out right.

 

Great insight, guys. This will be my first truck, and I didn't even know there was a "boot" for a cap. Boot, bed rug, insulation, etc., all good info.

 

http://www.pickupspecialties.com/index_images/CAMPER_BOOT_010.jpg

 

I have a first gen with a cap, live in CO and spend too much time in SD during the winter working. Wired in an inverter and found a direct wire 12v space heater that I mounted to the roof of the cap. It does a decent job bringing it at least up to the 40s-50s when its negative cold as f*$^ outside. Also installed an optima battery for deep cycle capability. Just be careful, the heaters suck a crazy amount of juice but I've run mine for a couple hours on the optima and truck still started fine.

 

In my experience, you're going to run into way, way more trouble from wannabe good Samaritans seeing your dog locked in your truck than you ever will with actually keeping the dog comfortable in your truck. These same people that take their dog out for a jog in the sun on a 100 degree day will absolutely lose their shit if they see a dog in a vehicle with its windows halfway down on a mild sunny day. You'll need to go way overboard making it look like it's not hot in there, and in the process you'll also provide an environment that's very comfortable for the dog, so I'd just shoot for that.

I figure there's few things in a dog's life more enjoyable than a ride in the back of a pickup, so don't deprive it of that! I think a cap is a great idea, and you could just reinforce the inside of the windows with some fence or screen, whatever keeps the dog from jumping out. Then in the summer just park in the shade and open all the windows on the cap. In the winter, a good insulated dog box mounted to the bed should be more than enough. If it's really cold and electricity is handy, Zerostart makes an excellent 900 watt space heater designed for car interiors called the Lil Buddy that would work great.

Disclaimer:
I know that leaving a dog in a car in the sun on a hot day is bad. OP is obviously not going to do that.