You leave us to guess at a tremendous number of things; you should try to provide as much as possible to give us something to work with.
Unhampered by facts, I will try.
First, were there no installation instructions?
Second, are all three non-RCA plug leads - the yellow, red, and black(?) ones all fiber optic leads? Most analog to fiber units I am familiar with would have, in addition to the analog (copper wire) inputs, three fiber optic leads: a fiber optic in, and a fiber optic out, and a copper "control" wire (possibly the yellow one) that turns the unit on or off as it is needed.
You are going to have to connect the RCA jacks to the outputs on the back of your head unit; then, you need to connect the FOB02 unit to your fiber optic amplifier. Because you have apparently already removed your original head unit, that could be more difficult. If you can power up the remaining portions of your audio system (you may not be able to), disconnect whatever fiber optics are going into the amp; the one that is lit up on the end is the input; the other is the output.
As for as identifying which is the input and which is the output on the FOB02 unit, unless your instructions cover it, or unless you connect the RCA jacks to your head unit and can turn it on via the control wire, I cannot tell you which connection is which. If you are able to get anything out of it, the fiber lead that is lit up on the end is the output, and it will go to your amp's input.
You can read more on how the MOST fiber optics system works, and what the leads do (and how to add to the connectors) at https://web.archive.org/web/2006090...elematics/416 HO MOST (CooksonI) 07-01-04.pdf
You will need to be able to make the fiber optic connections (DO NOT CUT ANY FIBER LEADS) - you do this either by replacing the wire from your old HU in the original fiber optic connector; or by using splicing devices (they are inexpensive, and available from your dealer).
Sorry the answer is so indefinite, but you really need to give us more to go on - and I hope your FOB02 unit came with instructions. None were available through the sites I saw selling them.