How do you install an engine block heater 2004 Chevy trail blazer V6-4.2L?

First check and see if there is a pouch located directly beside your battery. If there is a pouch it will contain a plug that is for the factory engine block heater. If there is no plug then you can pursue an aftermarket type. There are several types. If you want to use a freeze plug block heater, you'll need to have someone help you remove a freeze plug. That usually requires that you remove a lot of the peripheral equipment until you can get a clear access to one of the freeze plugs. Measure the freeze plug then go to an auto parts store and purchase a freeze plug block heater of that specific size. Once you have one that will fit into the hole, you can drill a hole in the freeze plug, then use a prying instrument to remove the old freeze plug. The block heater freeze plug usually goes in with a nut to tighten the freeze plug into the hole. BTW, you'll get a lot of antifreeze all over when you do this, and plan on taking your time since it will be time consuming.