Ever since I got this truck, i've had sporadic issues with both the GPS in my phone (wont display speed or refuses to get a signal). Also, seems I have the same issue with my TomTom unit. It doesn't matter where in the windshield I put either, they both lose satellite connection. Both units work great in my LeSabre. My Garmin Oregon 450t is accurate inside the truck, but it has a higher sensitivity shielded receiver. I cant understand why the truck is interfering with the devices? The only thing that makes me wonder is the aftermarket satellite radio. Its an older Kenwood unit, where the faceplate mounts up front, and the antenna and factory converter unit is mounted under the rear seat. The antenna sits on the cab back toward the cap.
I'm kinda stumped here. Not sure why this is happening, but would really like my GPS units to work in the truck. I use the satellite radio everyday, so I certainly can't rip it out. Any ideas?

the sat radio is a receiver so it isnt emitting any interference and if it was the modulator you could turn it off to see if that was the cause.

Puzzling. Perhaps you could determine if it is mechanical shielding or electrical interference by disconnecting the truck'* battery and seeing if the GPS works?

Is there any tint on the truck?

I'd say 99% of all types of radio equipment ALWAYS emit a harmonic of its receiving frequency. Considering GPS'*, sat radio, sat phones, AND radar detectors sit right around 2 GHZ, it is always a possibility. My TomTom will set my radar detector off if I put it right in front of it. Same goes with radar detectors next to others. They sometimes set themselves off.

Another weird story. I parked my car at my sisters house RIGHT next to her DirecTv dish. I mean 3 feet away. Now, my brother-in law being a sports fan lost one of his sports channels. He spent 2 hours with tech support trying to figure it out. Finally they asked him if anyone had parked a car with a radar detector next to his dish. Well, you get the rest.

The antenna for the Sat Radio isn't anywhere near close enough to the GPS unit though to cause problems. I could jump into explaining the inner workings of transmitter/receivers/transceivers but I won't bore y'all. Also, in your case you had a radar detector. Radar detectors are transceivers so they actually do send out a base signal as a reference. It'* how they know what kind of signal they're getting pinged by.

Anyway, this is why I asked if the truck is tinted or not. Since he'* having trouble with more than one device. If there is tint on the truck, it'* likely to be metallic based. If so, then its been proven that the metallics in window tint can affect GPS signal, cell signal, etc. That'* why they moved to ceramic based tints.

Then what'* the TomToms excuse for setting off my radar detector?

No tint other than the factory stuff on the back doors.
And it doesn't have OnStar in the mirror, so no interference there. The only thing I can put a my finger on is that radio setup. All the new radios are self contained. Run the antenna in, 12v power, and an audio signal out. The base unit for this radio is the size of a small amp! Has a fan, and a huge amount of wiring under the back seat. Plus its wired into the factory radio somehow, using a RF transmitter, since you still match the stations on either unit. I wonder if that is interfering with the devices? I'll have to try pulling the plug on the system and see if problems go away with the main unit not powered on.

Looking around the internet for this problem, it seems to happen fairly often. From what I read some people are using a secondary external antenna. If you can't determine the source of interference this may be an option for you.

Because GPS devices send out a small signal themselves, but its just around themselves so that'* why its only when its RIGHT there that it does it.
The RF transmitter isn't the problem. Its not in the same range as the GPS signal. FM radio is in the lower side of the spectrum, at no more than 108 MHz. GPS is in the SHF/UHF (can't remember which) at either 1.57542 GHz for the L1 signal and 1.2276 GHz for the L2 signal.

Do you have both your cellphone and GPS unit on at the same time? They could be fighting amongst themselves. I've had that problem with my iPhone and TomTom in the same car before.