Is there a way to hook up the horn to the the flashing light alarm on a 2005 Electra Glide, so that when the lights go off the horn will go off also, instead of having to buy the HD alarm system?

 

Can't help with the horn but you might do a search for "Radio Shack Siren". They're siren is around $5 and can be hooked up to sound off with the stock alarm system.
Good luck.

 

Yes there is.... I done it to my 03 Road King. What happens is, when you arm the alarm, the bike's horn will sound just as the indicator flashes on the dash. If the alarm is set off the horn beeps along with the lights. It does take some time and a little knowlege of wiring. If your intrested PM and I will walk you throgh it.
HiTech...