Hyundai: H290 Radio in a 2004 Elantra?

I am trying to install an H290 CD and Cassette in my 2004 Elantra. The
dealer said that this was not an option, but the Owner's Manual and Shop
Manuals both list this radio as optional equipment.

PROBLEM: I bought the H290 (actually, H-295YFU) to replace my H260
(actually, H265JU HECT-HMC). But with the volume control at max, the sound
is very quiet.

The H290 is stamped, 12 V, 2 Ohm, 5 A (CD and Cassette)
The H260 is stamped 12 V, 4 Ohm, 10 A (CD Only)

The Shop Manual describes three kinds of sound systems, but it doesn't say
which head unit goes with which system. I am confused.

1. Did anyone get a 2004 Elantra with the H290?
2. Is there a power amp that would be required with the H290?
3. Where would such a power amp go?
4. What about the mysterious 3-wire connector that goes into the H290. The
Shop Manual doesn't say what this is for, only that Terminals 1 & 3 are
Ground and Terminal 2 is "Signal". This connector, ETJA001K, is not
mentioned anywhere else in the book, as far as I can see.

My car dash is in 1,000 pieces. Please help so that I can put it back
together! Thanks so much!

--George

I found today that the H290 is externally amplified, so there's the biggest
issue. I don't think anything available on the Elantra has an external
amp, so there may be significant issues with trying to get an amp and
install on this vehicle.