Draggar, how 'bout antennas???

I still have the external antenna that is center-mounted on the roof of my van...I used it for my old Motorola bag phone. Now, that I've switched to a Nokia, I've had several people tell me that they can't hear me...I can hear them tho. I think that trying to talk on that hand-held phone inside the van with that little antenna isn't making the trip. Anyway, RatShack sells these cable abtapters that allows me to plug my old cell phone antenna into my Nokia. It definitely gives me a stronger reception indication on the little strength meter.
Tech question: should I be worrying about SWR's, etc. with using this other antenna? The hand-held Nokia only puts out 600 mwatts.
Also, I've seen these "passive repeaters" that mount on the window of your vehicle...are these things really effective?
Just need some good straight advice. I drive on the interstate highway system all the time and I need a good reliable set up.
TIA

I also worked for sprint, and honestly if you are in analog mode then they seem to work, but if you are in digital, they are a joke