it is the side tank. it is replaceable separately but not cost effective.

 

Make sure the mechanic doesnt get a cheap crap Autozone one. I used one in mine and the after about 12 months trans cooler blew up inside of the radiator causing my coolant and tranny fluid to get mixed together.

 

if you where mechanical, you could try JB weld coating the crack. No joke I did this to my old nissan 240sx radiator one time as a temporary patch.. But eventually over time it comes off.