Kim,
now that the US miltary has switched off the SA feature in the GPS system, you will find that the current level fo accuracy is pretty good for navigation.
DGPS can be used to increase this accuracy - usually the DGPS signal which is in a format called RTC104 [from my very rusty] memory. This has the 'corrective factors' to be applied to the information supplied to your GPS receiver by each the GPS satellites that are currently in view and being used to determine the GPS receivers location.

This corrective factor (or factors - at least 1 per GPS satellite) is then used to adjust the actual calculated lat/long/altitude the GPS receiver displays/outputs.

Most GPS receivers that have a DGPS input usally need to be given the corrective information in real-time via their DGPS input which is like a serial port [at TTL level].

The other point is that although the local Radio Station may output DGPS information on RDS, is the DGPS information transmitted in the clear [no encryption] or scrambled [with some encryption of the data].
If the latter then the DGPS information will have to be decrypted first.

In the old days [well say this time last year],in the US, DGPS broadcasts via RDS were routinely encrypted and only available to subscribers for a small annual fee [plus the right equipment].

Whether this is still the case I do not know. However more than one company in the US makes aliving from selling DGPS information over a RDS stream from a local FM station.