I 've been trying to find a game where you could customize your vehicles with different treads, bodies, and weapons. I don't remember it being super in depth, more of just a slider for each slot. I think the name of the game was also the name of the factory that you created units from, but I could be wrong there. If anybody has any idea what I'm talking about or could find some screenshots that I could recognize it from that would be great. I had it on a demo disc from a PC Gamer magazine if that helps any.

Perhaps http://www.mobygames.com/game/windows/warzone-2100 ?

It sounds like you're thinking of Warzone 2100. It's actually available as freeware (and open source) here:
https://wz2100.net/

Edit: Ninja'd!

Could also be Hostile Waters: Antaeus Rising. That does have the name of the factory (Antaeus) in the title.

Possibly Outpost 2. That has modular vehicle design.

Makes me think of Colony Wars 2492.

I doubt it's game you're thinking of (and I imagine one of the previous posters has already got it) but the Earth series has customisable vehicles.

Tanktics?

Could easily be BattleZone 2. Here is the video.

None of these look like the game I'm thinking of, but some of these look like something I need to play for my 6 weeks of holidays coming up lol. The art style for Tanktics looks pretty close to what I remember, but this was also ~20 years ago and games never look like what we remember them. Tanktics doesn't quite look right from the video's I've found on it.

If I'm remembering it right, you could go into the tank factory workshop thing from the main menu and create some tanks or you could do it while you were in a mission but it would take you out of the battle and put you in a full screen 'workshop' to make the tanks before going back to the battle. I wish I still had my old demo discs, there were some great games I'd love to play again.

Was it Z?
[url=https://en.wikipedia.org/wiki/Z_(video_game]https://en.wikipedia.org/wiki/Z_(video_game[/url])

Wow! That looks like a cool game! Here is the wishlist!

I certainly recommend Hostile Waters: Antaeus Rising, if you haven't played it. The UI is somewhat cumbersome, and the learning curve is a bit steep to begin with, but apart from that the game is excellent. You have a number of AIs with individual specialties and personalities that you can assign control of your units to, and at any time, you can take personal control of any unit you wish. I had a blast with it. In fact, I should play it again very soon.

Reminds me of Metal Fatigue.

Hostile Waters was a lot of fun! I'd discovered this game on GOG and couldn't believe it was almost forgotten by history! It's a shame that a sequel was never made.