To enable the authentication you are prompted for a username and password which is sent to the TACACS or RADIUS server. You can log into the PIX with TACACS or RADIUS and enable through TACACS or RADIUS with the same username/password because the authentication packet for enable is the same as the authentication packet for login.
pix(config)#aaa-server topix protocol radius
pix(config)#aaa-server topix host 10.31.1.41 cisco timeout 5
pix(config)#aaa authentication enable console topix
http://www.cisco.com/en/US/products/hw/vpndevc/ps2030/products_configuration_example09186a00807349e7.shtml#conf