I've never seen an oem car kit for the 7520. The only thing you could do would be to do a "roll your own" solution.
Use a generic phone holder, an OEM Car charger and either a bluetooth or wired headset or speaker/mic combo.
As for external antenna, Wilson Electronics might have one you can use. They have active boosters for IDEN phones. They don't list a compatible antenna plug on their web page, but they might have one compatible if you call. If not, they have one that clips to the existing antenna.
www.wilsonelectronics.com