The less expensive but very good alternative to a Jensen isolator is Edcor.

PAC is one brand of car stereo isolator that works and it's slightly over-wound, so it provides a slight increase to the signal level. It also has input and output labels. I had also used the ones from Radio Shack- if you still have one of their free-standing stores or a Hobby Town, they may have, or could order these (if they still offer it).

Here's a store locator-

[Link: radioshack.com]