I am truly amazed by my success at this diagramming business, but I wish for a rest now.
The word now is a preposition (intransitive prepositions like now are thought of as adverbs in traditional grammar). This preposition phrase functions as a modifier in both noun phrases and verb phrases. It is a temporal adjunct, meaning it gives us information about time.
Preposition phrases can freely postmodify nouns - a property which distinguishes them from adverbs and adverb phrases:
- the man in the park
- the elephant at the centre of the controversy
- the penguin in the tuxedo
- that silly billy over there
- a stitch in time
- one mistake now
- A rest now will be much better than a rest later.
And, of course, as we said earlier preposition phrases can modify verb phrases too:
- sing like an angel
- lean at an angle
- teach on Wednesday afternoons
- read at home
- play on your own
- come now
Now temporal adjuncts in the clause structure can usually come at the end or the beginning of the clause:
- On Wednesday afternoons I play tennis.
- I play tennis on Wednesday afternoons.
- I take a break at one o'clock.
- At one o'clock I take a break.
- I have a donut once in a blue moon
- Once in a blue moon I have a donut.
The Original Poster's example
If we consider the original example without the word now it becomes obvious that we cannot tell for certain what the word now is modifying just from the meaning:
- I am truly amazed by my success at this diagramming business, but I wish for a rest.
It is quite probable that the speaker wants to rest now. We also know for sure that the speaker is entertaining this wish now too. We can tell these two facts without actually using the preposition at all. Therefore a speaker could easily use now to modify either the verb phrase or the noun rest and the sentence would make perfect sense.
We can also see that the word now is in a position which is ambiguous between the two readings, because it could be postmodifying rest or coming after the verb phrase wish for a rest.
Because of this we cannot tell for certain whether the clause means:
- A rest now is what I wish for.
Indeed, the two sentences above provide independent evidence that both interpretations are possible. The first sentence shows that a rest now can be a noun phrase in its own right, therefore supporting the idea that this is what we are seeing in the original sentence. On the other hand, if now is a temporal adjunct in the original, then we would expect to be able to front it to the beginning of the clause. This is exactly what we are seeing in the second example above.
It seems, therefore, that the Original Poster is completely justified in analysing now as a modifier of rest, although it could just as easily be a modifier in the clause.