A 337 is an application for a Major Alteration.
It needs to be supported by Approved Data.
What constitutes approved data can be found in
AC43-210.
There are many things which can be used.
Some are high grade e.g. an STC for the specific S/N range, or an AML STC, and nobody will argue with those.
Similarly, using a DER to produce Approved Data, and delivering an 8110-3 design package, is a pretty good way for an installer to cover his 6 o'clock. This can be awfully pricey - I have seen a quote from a US DER for $10,000 for basically 1 or 2 days' work knocking up a wiring diagram from the back of installation manuals.
Then you get onto other acceptable categories of approved data, but they are less "good" and more open to opinion, especially if something went wrong. You can use the
contents of an STC (but not the STC # itself) for a different type (but somewhat similar) aircraft, you can throw in all the installation manuals you can find (if they are FAA approved), etc.
AIUI, European installers are basically limited to using high grade approved data i.e. an airframe S/N range specific STC, an AML STC (and hey guess who owns that particular universe.... Garmin
), or use a DER to produce it. This does away with the need to get the FAA to pre-approve the application.
The routes which involve the traditional pre-approval method using a US FSDO can be done but the official FSDO covering Europe is not doing it anymore. I don't know why - may be some "funny business" going on in there. But there is no fundamental reason why any US FSDO cannot do it - you just need to get them to deal with you.