New Radio reception is not as good....

The first tell tale sign is to check is the AM radio. AM radio needs a decent antenna whereas FM can work with a coat hanger

Many new cars have radios antenna built into the windscreen and these antennas have their own amplifiers and so the task is to repower them.  The methodology is quite simple and a multimeter needs to be used.
With the old unit still connected you simply tap each wire on the back of the plug and look for 12V power wires.  Then when the new unit is connected test these wires and one of these wires will not longer be powered and this is the antenna amplifier wire.   Run a accessories wire to this.  Some models will have an AMP or ANT wire which can be used.   If not check if your cigarette lighter runs only on accessories and use this.

Other known problems can be the antenna plug in your car may not be the standard Motorola sizing, but may look similar.  

Also check the earth to the antenna and the stereo itself.

Lastly when in doubt connect a standard cheap rod antenna.

Depending on your model Google is also a good source of information