New Transmission

So the saga continues. My transmission is dead. They're coming to tow it today. I found a used tranny with 75k miles on it at a junkyard for $300. And the guy is charging me $300 to install it. It seems the radiator that I just installed less than a month ago was bad. The transmission fluid was mixing with the coolant. I stopped at a shop by my house, they told me that the only way that can happen is that the trans cooler inside the radiator is bad. (which is believable). So they install a aftermarket trancooler, and a new radiator for me. On the way home the lines going to the aftermarket cooler blow off and bye bye transmission. When I look in the radiator, there's transmission fluid in it again. Was that just residue from before that they didn't get out of the system? The odds of me having 2 radiators go bad (the 2nd was brand new from NAPA $170.00) are very slim don't you think?

I'm sure the thing was on its last leg anyway, and I guess it's a pretty good deal for installing a transmission. My question for you guys is...Is there any other way that trans fluid can mix with the coolant.? It just didn't seem like they did a ton of diagnosis on a pretty significan problem. So I defer to the experts...What do you think?