Installing a game that uninstalls

Hello everyone,

I’m facing a problem that I’m having trouble understanding.
I have a game (Ghost Recon original), as well as its two add-ons. I’m able to install the base game without any issue and play it. However, when I try to install one of the two add-ons (“Desert Siege”), it only offers me to play the game or to uninstall Ghost Recon. The second add-on, on the other hand, works fine.
In fact it’s as if Desert Siege was already installed and that there were only options to either play it or uninstall it. But Desert Siege isn’t installed because, when I launch the game, it’s impossible to have the missions from this add-on and it doesn’t appear in the game files either.
I’m wondering how to install the Desert Siege add-on.

If you have anything to clarify my problem, I would be grateful.

Thanks in advance!