Depending in the year of your car you may need to purchase the xm module that goes near the OEM car amp. I checked into doing that a few years ago and I think it was around $300. There's a DIY here in CL.
06 IS350: (Original Owner)3M Clear Bra, 20% Tints, Rear Spoiler, 4300K Phillips HID, K&N Typhoon Air-Intake, "Lexus" Caliper Decals, Blue Illuminated "Lexus" Door Sills, OEM Sport Pedals, Tanabe Medalion Exhaust, 4080 box w/ Kenwood 12" pushed by Kicker 500.1 Amp, Vossen VVS085 20X9 235/30/20 front & 20X10.5 275/30/20 rear, JDM Style Side Mirrors w/ LED Turn Signals, Megan Coilovers
05 Toyota 4Runner: (Original Owner) V8 Limited Package
I am not saying you can't get it. You can purchase a second hand (used) unit from a yard or on here if they ever show up. What I am saying is that it is no longer made by Lexus and finding a used one is rare. Especially because you are missing the mounting bracket which seems to be the hardest part to find. The unit is broken into two parts, reciever and mounting bracket (cables included). I wish you luck in finding one but your best option is to purchase an aftermarket unit. Personally I would like the OEM route also, so I am just waiting.