Rear defroster question

I'm in the process of installing the rear defroster and I really don't want to get rid of one of my 6x9 speakers in the back to install it. Do you guys think it would be a bad idea to run longer studs through one of the speakers and just install the blower motor below the speaker? The draw back would be the air intake would be coming from inside of the trunk instead of pulling the air from the interior. I don't have a rear battery so the risk of outgassing isn't there but I'm not sure if enough exhaust gets back there to render it unsafe to install that way. I do remember riding in trunks all the time as a kid so it can't be that unsafe right? :D

Here is a picture of a correctly installed rear defroster that I found from another member on here for reference.