Vintage receiver problem: low volume.

I think I have a classic old receiver problem and need some help.
I picked up an Olympic Opta 52805 stereo and record console tonight. The record player in it was wrecked- no cartridge, no platter and not really fixable to my eyes without impossible to find parts.
But the reason I picked the thing up for $25 is that the old tube amp works, and it sounds really big and round. I love the tube amp tone.
Now, I want to be able to plug in various devices into it. I figured the best way to do that would be to trace the wires back from the tone arm and figure out which wires connect to the receiver.
I did that and I wound up with two banana plugs running into a common cable.
Eventually, I want RCA cables running into a multi-device switch I have laying around, but I was so excited tonight that I cut the earbuds off a pair of headphones and tried to splice the wires into the receiver and plug the other end into an old mp3 player.
I was able to get the tiniest bit of signal through, but it was just barely audible.
Obviously, this is a long LONG way from where I want to end up and I'm going to solder proper connections to the banana plugs etc.
Ultimately, I'm wondering what the best way to do that will be and whether I'll need a preamp or some other device.
My thought was to get some RCA cable and solder together the two grounds from the cables into the ground, and solder both positives (both channels) into the one positive.
I'm a little new to all of this, so please forgive me if I've said something really stupid. I just think I've found a nice old receiver and would like to plug my music into it, one way or another.
It appears that the cable you have spliced into led to a phono preamp. It would seem that you need to find another avenue (line level) into the amp in order to get proper volume .

Regards, Pigtown. Ceramic cartridges would have output from 25 to 600(!)mV.

Thanks for the help. Looks like I got lucky here. When I got the unit, someone had already tried to splice something into the preamp. My theory is that they did so at line voltage and blue out the existing pre amp. When I tried to hook my record player direct, I got very little sound. When I tried to do it through my own pre amp, I got sound. From their, I was able to use an input selector and now I have four inputs going to the original phono input.
As a quick aside, I found this website really helpful (although admittedly a little out of my depth) on solving the whole stereo out to mono in problem.
Thanks again for the help all.