Eric Marciano's Radar Gauge is used by Johan Peeters for his F-16 Panel and by Ronald Cachemaille for his "Back to the Future" DC3 Panel and his Mobile Control Tower. 
I have attempted to register it using the Freeware Application Key for F16.gau: DSVT JE5S 6DUO.  
I followed the "Register an Application" process in the User Guide and the registration was accepted but re-starting FS2004 resulted in the "unregistered" message.  
In the FSUIPC.log the pertinent entries are  
Aircraft="F-8E Crusader" (with F-16 Panel),  
Aircraft\Douglas_DC3\Douglas_DC3.air (with BTF Panel).  
FS2004 is Version 9.1 Build 40901.01.  
I would really like to make this gauge functional.  
Regards,  
David  
Contents of FSUIPC.log  
********* FSUIPC, Version 3.44 by Pete Dowson *********  
Running inside FS2004 (FS9.1 CONTROLS.DLL, FS9.1 WEATHER.DLL)  
User Name=""  
User Addr=""  
FSUIPC not user registered  
WIDEFS not user registered, or expired  
Module base=61000000  
ClassOptions: UIPCMAIN=FF7F, FS98MAIN=FF7F, FS2KMAIN=FF5E  
WeatherOptions(Orig)=0000B027[0000B027]  
InitDelay: 0 seconds  
WeatherReadInterval=4  
LogOptions=00000001  
DebugStatus=0  
7070 System time = 16:41:02  
7070 \\DAVID\C\SIMULATORS\FLIGHT SIMULATOR 9\  
7073 System time = 16:41:02, FS2004 time = 12:00:00 (00:00Z)  
12811 FLIGHTS\OTHER\FLTSIM.flt  
13103 AIRCRAFT\c172\Cessna172SP.air  
13157 Aircraft="Cessna Skyhawk 172SP"  
43712 AIRCRAFT\F-8 E_HMC\F8 Crusader.air  
43766 Aircraft="F-8E Crusader"  
51012 C:\My Documents\Flight Simulator Files\UI generated flight.flt  
51694 Clear All Weather requested: external weather discarded  
53696 Illegal read attempt: offset F000, size 2 [P0]  
53696Program or module not accredited for use with this unregistered FSUIPC  
55368 ### IPC Message processed in 1673mSecs ###  
55947 Advanced Weather Interface Enabled  
92945 Traffic File #15 = "scenery\world\scenery\traffic030528"  
125795 AIRCRAFT\Douglas_DC3\Douglas_DC3.air  
125863 Aircraft="Douglas DC-3 Paint2"  
280817 System time = 16:45:35, FS2004 time = 16:43:41 (23:43Z)  
280817 *** FSUIPC log file being closed  
Memory managed: 2 Allocs, 1082 Freed  
********* FSUIPC Log file closed ****