I had the same problem as gillberry, & it was the fag lighter that was to blame. Replaced that & the horns worked fine!
As forthe horns, the horns on both my Vitesse & my old 2600 VDP didn't work, & I traced it to dirty contacts on the horns themselves. They are in a truly stupid place & get coved in all kinds of crap & get dirty quite quickly. Give them a clean with some emery cloth or sandpaper & squirt a bit of WD40 into the connectors & they should work OK.
Failing that, get some air horns!