I have no issues with reception on my aftermarket radio, and its the cheapest pioneer they sell at walmart.
Did you plug in just the wire that that goes to the mast antenna??
Info from 2.5.5 Antenna Connection section at:
http://ae64.com/AudioInstall-2.htm
"The SVX and factory radio have two antenna cables.
One cable goes to the telescoping power antenna. The second cable connects to the rear window defogger (which doubles as an antenna).
The factory radio uses a feature called "Diversity" for better FM reception — it picks whichever antenna is getting the stronger reception at any moment. How can the rear window defogger make that much difference? You might have experienced this in a car with a single antenna: While slowly rolling up to a traffic light, the reception on a particular FM station drops suddenly or gets lots of static. Just moving the car forward by a foot or two makes the signal return. With Diversity, in many such situations the two antennas are far enough apart for one of them to "see around the obstruction."
Unfortunately, receiver manufacturers have stopped including Diversity. According to the folks at Crutchfield, the last model they sold that had it was in 2001. What this means to you? Unless you've bought an older receiver that has Diversity (i.e., has two antenna cables), you'll only need to connect one antenna to your receiver.
Don't use a Y-adapter to connect both antennas to a non-Diversity receiver. That will give you worse reception than just using the SVX's power antenna alone (it's an "impedance-mismatch" thing).
Make your antenna connections:
Take the SVX antenna cable that ends with the male Motorola plug (fatter diameter cable, with the pointy end). This is the cable that comes from the telescoping power antenna. Connect this cable to the female end of your antenna extension cable."