we have just bought this viv Popup Window 510409

and have got a heat mat size of mat is 16"x12" and a thermostat.

1 - do i put the heat mat on the back of the viv on the outside or inside of the viv???

2 - do i put the heat mat on the bottom of the viv inside under the soil???

3 - does the thermostat have be on the heat mat or in the soil????

Please help as my hubby and i are having a dispute

Wow, you have spent a lot of money on your GALS! Not a bad thing, but I just keep mine in a £5 plastic storage box!
The only thing with that viv that could be an issue is the sliding doors. If one of your snails is busy climbing on the doors, you might not be able to open it until it's moved, without trapping it.
And it doesn't look like the substrate could be deep enough too?

You can't put the heat mat on the inside, because the environment is far too humid and wet.
If you put the heat mat underneath it will warm to substrate (I prefer this) but if you put it on the side, it will warm the air.
Either is OK, it's up to you.
The only thing with putting it under the substrate is not to let the substrate dry out through.

I don't put the thermostat prob on the inside. Just because it would get caked in slime and soil and all sorts. I just stock it about half way up on the outside of the viv, over the heat mat.
All you really need to do is make sure they don't get too hot.

In winter, snails spend more of their time under the soil. DO NOT put the heat mat beneath the soil. Put it on the back of the viv, above substrate level. Thay can actually cook in their own substrate if heated from below. Also, heat mat should only be 3/4 siz of the tank so they have a cooler end to go to.

A snail will sense they are feeling too warm and will move to the cooler end of the tank.

Plus, all heat mats should be on a mat thermostat, so what you say will never happen.

The only downside to putting the mat under the tank is the substrate drying out, which of course is very easy to keep on top of.