Glad you got this sorted and well done for taking the alternator off and putting it back in, looks like there is a lack of space in Caddy's same as Shalaxies
Top Tip: Use Mole grips on that lug when loosening the tensioner. Much easier than using a spanner, allows you to pull on it with one hand while the other hand slots in a allen key to lock the tensioner with ease.
A bit late, but maybe it will help someone