I’ve researched “heater” and “autosteer headlights” and appiled them to the basic model of the sedan car.
The Heaters are not being installed and the autosteer headlights are not in the item list of parts, so they are not being installed either.
Cheers,
L
Edit: Having further played the game it seems other researched items are not being installed (pic below, follow link):
http://steamcommunity.com/sharedfiles/filedetails/?id=1087382610
Further Play on this bug shows that items are being installed in the Electronics bay but not any in the Accessories bay,
bit of PITA due to you can’t sell any decent cars for decent cash 
Has anyone else had this issue?
I’ve just found an example of this, which I’m fixing…relating to aircon.
Can you verify that:
The slots in the factory world have the upgrades installed AND
The cars were not made before those upgrades were installed or before the up[grades were added to that vehicle design.
There is a separate bug relating to you being able to select climate control as an upgrade without aircon…which I’m fixing…
All the cars where built after the edit to the car design took place (for certain).
After much more playing, and researching everything in the tech tree and putting in place all the upgraded expanded slots, all the accessories are fitted.
I think I was expecting the basic starting slots to be upgradable as the expanded slots are. The Electonics slot is upgradable and I was expecting the Accessories slot to be upgradable too.
I find it odd that the basic “add accessories” slot doesn’t actually add any accessories from the tech tree and this was the problem I was having in my original picture above. This is detrimental at the start of a game when you need to be able to produce cars with accessories to make them more appealling and in order to make cash to afford the expanded slots.
This seems to be highly restrictive and don’t know if this a feature or a bug now 
Its by design - The Basic starting slots are really only good for getting your factory going, and getting some research under your belt. With the starting slots, there are only some electronics, and Air Con improvements you can make.
Once you research the specialized slots, you have to reconstruct your line, or build a second one. The Main reason is if you were to add in the improvements to the basic slots, each station would start taking over an hour each slot to process. Not to mention the sheer volume of parts needed at each import slot