This circuit by vk3ye requires three transistors adds a range of shortwave bands and a BFO so you can receive Morse code and SSB. You need to do a search for 'parkerp' and 'projcan' or you can search for RUSHCONV dot pdf (This converter allows an ordinary AM car radio to receive 15.42 MHz (WRNO). An (in) famous talk show is transmitted by this station. The unit should be constructed in a metal box with antenna connectors compatible with the car radio)
There are kits from Ramsey electronics, Vectronics and even MFJ but home made is the cheapest option.
Like this circuit using an ne602 or equivilent on techlib dot com forward slash HAMRADIO forward slash HAMRADIO forward slash CONVERT dot txt (all lower case)
The text description is below.
C1 Optional capacitor. Use 150pF with 5 Mhz crystal, or 47pF with 8 Mhz
If using crystal ò 10 Mhz omit this capacitor.
C2 33 pF
C3,C5 220 pF
D1 Zener diode between 5.1v - 6.2v (2N4691 or equivalent)
R1 1K resistor
U1 NE602N Frequency Converter IC
T1 10.7 Mhz miniature IF transformer (green core)
XTAL 5 Mhz or 8 Mhz crystal, or other values (see text file XTAL.TXT)
One for each band you like to listen to
8-pin DIP socket (for U1)
J1,J2 RCA jacks [J2 is optional, see note 3 below]
Motorola plug (optional. If installing converter externally, use for
connecting cable to car radio's antenna jack). Radio Shack parts #274-711
14-pin wire-wrap socket and 14-pin DIP headers for mounting crystals
Optional (see file XTAL.TXT)