Hi folks a newbie here but thought I share my experience with others.
I bought a used and bog standard Astra H Club 1.3 CDTI Estate last year, just out of manufactures warranty period. A few days later I found the AM radio seemed to be totally buggered although the FM section worked fine. Don't use AM that much like most I guess so decided not to worry to about it.
On searching google found this forum amongst others where people seemed to have the same problem. I Eventually ruled out the radio when I found the antenna connector just inside the rear headlining near the tailgate. By undoing the connector and touching a length of wire to the aerial lead now had AM reception! So it seems the antenna is at fault. Vauxhall want £35 for just an antenna base. This is an amplified antenna base hence the hefty price.
Being a skinflint I decided to remove the base (a bit tricky to say the least) and have a look inside. There's a little pcb inside and a fair bit of electronics given its small size. Looked at getting an after market amplified antenna but none of those come with the unusual FAKRA connector vauxhall are using and they also need a seperate 12v supply to power up the amp, seems vauxhall must send the 12v supply down the center conductor of the aerial lead from the radio!
After some trial and error, eventually found I got reasonable AM reception by removing the center conductor of the aerial lead from the pcb and then doing the same with the red wire connecting the antenna base to the pcb and soldered the 2 wires together. This removes the electronics from the antenna circuit, this is now an unamplified antenna but Hey Presto now works on both AM and FM.
I live in the West Midlands so radio coverage is quite good here. I can now tune into to atleast 6 AM stations where as before I got absolutley nothing. It does mean entering the manual tune function and storing each station as a preset. The auto tune mode only finds 1 station on 693 khz but the others are easily scanned manually. Signal strength is obviously lower than intended but atleast the radio now works on AM! This may not suit others living in very bad reception areas however.
Hope this might come in useful for others, but you'll need to be handy with a soldering iron. If anyone wants help just ask and I'll do my best. Please bear in mind that my vehicle is out a warranty and yours may not be. Be very careful with the headlining as there isn't a lot of room and its easily creased.