For a W124 these bulbs you mention are used for "city lights" in the european spec cars. If you have a set of euro headlights you can see the cut-out for the citylights inside the actual main headlight reflector. They come in two versions...the push in type which is glass base is 5w5 and the metal base type is tw4. The glass type is also used for the side marker lights in the w210 chassis as mentioned.
You can pick up philips blue vision bulbs 9004 at Kmart
if you can find it in stock
for the main headlight. The blue vision are not as white as I think you are looking for, I know cause I run them on my euro w124. I also modified my euro city lights to run as a side marker light, when I signal I get white-orange-white-orange. I read somewhere that someone was trying to do the same thing to us-spec benz with euro lights and I don't know if they were successful.
If you are looking for pure white light you may have to go japanese (PIAA, Raybrig, Polarg, Nokya, etc) and they're not cheap. I am also on the look out for white lights in H4 format. I don't want something harsh just mild and white.
Hope this helps some....