When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
On my e99 250 the plug is right behind a bumper opening on the driver side, very convenient. Element is in the block nearby, but that's not what you're asking, I think. I believe they are stock on almost all 7.3's -mine was bought in GA, but yours may have the cord removed or something. Those are available.
Could be dealer added in '99. Best way to find it is to look at the oil filter housing and see if there is a fitting with a cord coming out of it. If so, follow the cord to find your plug.
If there is just a blank fitting with no cord, then you can install an OEM unit in that spot and run the plug where you like.
Lay under the driver's side of the engine and look straight up to the oil filter housing, you should see a red cord that comes out of the housing and goes forward to the bumper area. Look at this.