Hello and welcome to car-seat.org.
There is no top tether anchor for the second row center nor a retrofit for the center, unfortunately. Only the two rear outboard (passenger side and driver's side) vehicle seating positions have factory equipped top tether anchors in the 2001-2004 Grand Cherokee models, located in the vehicle ceiling near the rear courtesy lamp/interior dome light, and there's no tether anchor retrofit part listed for the center in the LATCH manual.
Seeing as you're apparently in Canada where you're legally required to top tether forward facing harnessed carseats, assuming you plan to keep the vehicle long term and ultimately have 3 forward facing harnessed kiddos riding in it, I recommend looking into alternative vehicles with three top tether anchors instead.
If your heart is set on a Grand Cherokee, the 2005 and newer Grand Cherokee models have 3 factory equipped top tether anchors if you'd consider a 2005+ model instead. On the 3 across aspect, you can peruse the forum's Successful 3 Across thread at
http://www.car-seat.org/showthread.php?t=33226 to get ideas of what's workable in various vehicles and the Successful Radian install thread at
http://www.car-seat.org/showthread.php?t=18235 can also give you input on vehicle compatibility for the Radian. Having 3 Radians already pretty much gives you a head start for 3 across, provided that the Radian is compatible with the specific vehicle model.