True for non hid bulbs but hc3 and h3 are the same for hid bulbs as they both receive + and - through the ballast. regular h3 bulbs are grounded through the base(metal part holding the glass) and the power is fed through the single wire. hc3 have 2 prongs one + and one - but the base is the same for both bulbs.
Noobie240 You will notice that your fogs have been modified to use H3 bulbs instead of HC3 using this meathod
http://zilvia.net/f/tech-talk/102925-official-s13-dual-projector-hc3-h3-foglights-maybe-even-low-beam.html
so if you need to replace them use H3 bulbs
On a side note Noobie240 if you want the stock silvia grille and the vent cover for the hole in the front bumper on the drivers side that belong to that front end PM me and you may even get a story out of the deal